Furnace heating element

ABSTRACT

An elongated electrical heating element embodying a ceramic core for use at high furnace operating temperatures. A metal rod extending through the heating element is post-tensioned by means of compression springs disposed at its outer ends, maintaining the ceramic core in compression to preclude fracture of the core throughout the entire operating temperature range.

United States Patent 1191 [111 3,846,621 Roos 1 Nov. 5, 1974 [5 FURNACE HEATING ELEMENT 3,548,159 12/1970 Ellstroem 219/552 X [75] Inventor: Paul W. Roos, Marlboro, Mass. FOREIGN PATENTS OR APPLICATIONS [73] Assignee; BTU Engineering Corporation, 382,751 11/1932 Great Britain 219/550 North Billerica, Mass 449,632 7/1936 Great Brrtam 219/550 87,930 5/1921 Switzerland 219/550 [22] Filed: Sept. 21, 1973 53,003 8/1933 Norway 219/550 1 pp No: 399,629 115,882 7/1926 Switzerland 338/316 Primary Examiner-Volodymyr Y. Mayewsky U-S. Attorney Agent or Firm weinganen Maxham &

[51] Int. Cl. H05b 3/02, H05b 3/06 [58] Field of Search 219/388, 535,536, 537,

219/544, 550, 552, 553, 19 HU; 338/316; [57] ABSTRACT 13/22, 25 An elongated electrical heating element embodying a ceramic corefor use at high furnace operating tem- [56] References Cited peratures. A metal rod extending through the heating UNITED STATES PATENTS element is post-tensioned by means of compression l 514 857 [1924 Maclnnes 219/550 X springs disposed at its outer ends, maintaining the ce- 1 670 437 5/1928 Campbell: iiiiiiiii 219/550 X ramic core in compression to preclude fracture of the g 5/1932 Conrad 338/3, X core throughout the entire operating temperature 1,935,163 11/1933 Parsons 219/550 X range. 2,571,422 10/1951 Cole et a1. 338/316 X 3,471,682 10/1969 Hisey et a1. 219/388 6 Clams, 4 Drawing Flgllres 1 FURNACE HEATING ELEMENT FIELD OF THE INVENTION This invention relates to heating elements for electrical furnaces, and more particularly toa ceramic core heating element which is maintained in compression during operation throughout the entire operating temperature range.

BACKGROUND OF THE INVENTION In electrical furnaces employed in the heat processing of materials and products at high temperatures, the heating coils are typically supported by ceramic cores such as grooved plates or tubes. The use of ceramic cores of this kind involves a significant limitation in the length of span of such members since, although ceramic materials have good compressive strength, they have relatively low tensile strength. Across an elon gated span of ceramic material, such as mullite, even at room temperature, the upper portion of the crosssection of the span is under compression and the lower portion of the cross-section is in tension. The tensile stress increases at higher temperatures due to the natural tendency of the material to sag. Particularly in spans extending beyond approximately 2 feet, sagging of the ceramic core, which occurs at high temperatures, becomes unacceptable and often results in fracture of the core due to tensile failure.

One proposed solution. using an arch-shaped ceramic core, has been found to be impractical due to the fact that the thermal mass of a suitable arched member is of necessity too large to permit the'element to respond sufficiently quickly to temperature changes.

SUMMARY OF THE INVENTION According to the invention, the ceramic core of an electrical heating element is provided with an internal axial metal rod. Springs supported on the outer ends of the rod are adjustably compressed against the respective ends of the core, thereby placing the rod in tension and the core in compression across substantially its entire cross-section. Sufficient compressive force may be developed to ensure continuous compression of the core despite expansion of the metal rod, even at its highest operating temperatures. By virtue of the compressive force applied to the core, sagging and possible fracturing of the ceramic heating element under high operating temperatures is substantially precluded.

DESCRIPTION OF THE DRAWINGS The invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a general diagrammatic representation of a furnace showing heating elements constructed according to the invention supported therein;

FIG. 2 is a partially cutaway elevation view of a heating element constructed according to the invention;

FIG. 3 is a sectional view of the heating element of FIG. 2 taken at the section lines 3--3 of FIG. 2; and

FIG. 4 is a diagrammatic view of the arrangement of electrical heating coils employed in the invention.

DETAILED DESCRIPTION OF THE INVENTION Heating elements resistant to fracture during operation at very high temperatures and constructed according to a preferred embodiment of the invention are shown supported in a furnace diagrammatically represented in the FIG. I. An elongated furnace chamber 20 is enclosed by thermal insulation usually in the form of refractory brick 22. A conveyor or other suitable mechanism 24 is provided for moving a work product through chamber 20 at a desired speed. A series of transverse electrical heating elements 26, supported at their respective ends in openings 28, extend through the furnace above conveyor 24, and provide desired heating of chamber 20.

Heating element 26, shown in greater detail in FIGS. 2 and 3, includes a generally cylindrical ceramic core 30 formed with flat end surfaces perpendicular to the longitudinal axis of the core, a cylindrical axial bore 32 extending therethrough, and a plurality of longitudinal generally cylindrical channels 34 open to the outer surface of the core, and evenly distributed about the circumference of the heating element. A cylindrical metal rod 36 is disposed in bore 32 with threaded ends extending beyond respective ends of core 30. A compression coil spring 38 is fitted about each end of rod 36 which extends beyond the ceramic core. Each of the springs 38 is retained between a respective pair of similar cup-shaped washers 40 and 42, and, as illustrated, each washer 40 is formed with a smooth, flat perforated disc-shaped portion, whose diameter is at least equal to the diameter of the ceramic core 30, and an outwardly directed cylindrical retaining flange. Good contact is thus assured between the abutting surfaces of the core 30 and each washer 40 for the uniform transmittal of forces. Each similar washer 42 faces inward and, as illustrated, is retained in position by a nut and lock nut combination 44. When nuts 44 are tightened on the respective threaded ends of rod 36, springs 38 are compressed, placing rod 36 in tension. This action, in turn, applies a continuous axial compressive force longitudinally to washer 40 and thereby to core 30 substantially uniformly across its entire cross section. By sufficiently compressing springs 38, compression of core 30 is maintained even during limited expansion of rod 36 under the highest predetermined temperature conditions.

Core 30 is typically formed ofa high temperature ceramic such as mullite which, as is known, has excellent compressive strength but relatively poor tensile strength. The ceramic core may be a unitary length of mullite, but, where long heaters are required, core 30 may be formed of a plurality of separate aligned axial sections held under compression by the arrangement shown. When more than one section is employed, the separate sections may be cemented together and aligned by hollow cylindrical ceramic plugs 45 which are securely fitted and cemented into one or more channels 34 shown in FIG. 2. The cement under these circumstances serves to insure transmission of the compressive force uniformly across the entire cross-section of the core, even if the abutting surfaces are not perfectly flat.

The electrical heater coils employed in the heating element of this invention are illustrated in the drawing as divided arbitrarily into three independent transverse sections, permitting the temperature of each section to be controlled by control of the individual heating currents. A typical heater coil arrangement is shown in FIG. 4, namely, for helical coils of a diameter snugly fitting channels 34, are serially connected by leads 52, terminating in terminals 56. As shown in the drawing,

the four helical coils 50are disposed in the lowermost four adjacent channels of core 30 for most efficient radiation toward the product (not shown) on conveyor 24. The upper two channels are used for terminal leads which as shown may be confined by hollow plugs 45. The interconnecting leads 52, which are employed to provide electrical connection between adjacent heating coils 50, extend between adjacent channels 34 along a double notch 54 formed therein. Terminal leads 56, which as noted, are provided at the respective ends of the serial arrangement of heating coils 50 are disposed in channels 34 and extend beyond the insulating wall 22 of the furnace for external electrical connection to the appropriate power source (not shown). The heater coils and leads may be made of nickel-chrome alloy or an ironchrome-aluminum alloy. in the practical embodiment shown, the three heating coil sections are separated electrically from each other by generally spherical ceramic beads 57, which may also be mullite, inserted in channels 34 between adjacent heating coils 50. The terminal leads 56 of the two heater coils sections adjacent the respective outer walls of the furnace are disposed in two channels 34 as they extend beyond the furnace wall. The electrical terminals 56 of the heating coil group centrally disposed in core 30 extend along thetwo upper channels 34 and through central openings in respective plugs 45 to a point on the outside of the furnace walls for external electrical connection.

Rod 36 is typically formed of an iron-nickel alloy such as lnconel or lncoloy when operation of the heating element at temperatures up to 700 is contemplated. For higher temperatures to l,600C, certain alloys of molybdenum may be employed if a suitable reducing atmosphere is supplied to the furnace. A ceramic material, such as alumina can typically withstand temperatures up to approximately l,900C. The springs 38 may be formed of steel and it is to be noted from the drawing that since springs 38, as well as washers 40 and 42 are outside the outer wall of the furnace. they are not subjected to the extreme furnace temperatures.

According to the invention, the composition of rod 36 and the characteristics of spring 38 are selected to provide continuous compression of the heating element despite expansion of rod 36 at high operating temperatures. The material used for rod 36 is selected to retain its tensile properties at a temperature which is higher than the operating temperature of the heating element.

In view of what has been shown and described, it may be appreciated that a heating element constructed and operative according to the invention may be suspended at or near its ends in a furnace of desired configuration. At operating temperatures where sagging of the ceramic would normally occur, the core is maintained under a continuous compressive force across substantially its entire cross section by the post-tensioned rod, associated springs and flat washers, thereby limiting sagging of the ceramic core and preventing fracture thereof.

It will be apparent to those skilled in the art that the invention can be embodied in a variety of heating element configurations to suit particular heat processing requirements and furnace designs and various modifications and alternative implementations of the invention will occur to those versed in the art. For example, in alternative embodiments, a single spring 38 and washer arrangement may be employed at one end of the core while at the other end of the core a'eompara- -ble flat washer welded to the rod abuts the confronting end surface of the core. Accordingly, it is not intended to limit the invention by what has been particularly shown and described. except as indicated in the appended claims.

What is claimed is:

l. A heating element suitable for horizontal suspension by support at its opposite ends within a furnace operative in a predetermined temperature range comprismg: j I

an elongated ceramic core having a plurality of longitudinal channels formed in the peripheral surface thereof and including:

at least one elongated unitary core section having flat end surfaces oriented in a plane perpendicular to the longitudinal axis of said core;

said at least one elongated core section when supported as aforesaid at temperatures within said predetermined temperature range being predisposed to sag and to place the lower portion thereof into tension;

electrical heating coils disposed longitudinally within said channels;

electrical connecting means for connection of said coils at said opposite ends to an external electrical power source;

a metal rod having the characteristic of retaining its tensile properties throughout said temperature range, said rod being disposed on the longitudinal axis of said core and extending beyond the ends thereof;

retaining means including perforated end pieces having flat faces confronting the respective flat end faces of said ceramic core and having an outer diameter at least equal to the cross sectional diameter of said core;

at least one compression spring disposed at at least one end of said rod and beyond said perforated end pieces;

spring retaining means including fastening means engaging said rod for maintaining said at least one spring in compression;

said rod being maintained in tension by compression of said at least one spring to thereby maintain the ceramic core continuously in compression throughout the entire cross section thereof during operation of said heating element at temperatures within said range whereby tensioning and subsequent cracking of said at least one elongated sec- I tion is prevented. v

2. The heating element of claim 1 wherein said at least one compression spring comprises first and second compression springs disposed beyond respective end pieces at said opposite ends for providing a uniform compressive force to said core from said opposite ends.

3. The heating element of claim 1 wherein said ceramic core is formed with a central bore extending longitudinally therethrough and said rod is disposed within said center bore of said housing.

4. The heating element of claim I wherein said elongated ceramic core is formed of a plurality of similar sections maintained in rigid axial alignment by said compression thereof.

5. The heating element of claim I wherein 3,846,621 5 6 said electrical heating coils are formed in indepenlike crss-section cemented together at the abutdemly *Tnerglzable Sections disposed lqngiwdilwlly ting ends thereof to insure transmission of said rcaglnntmg control of temperature axially of san compressive force uniformly across the entire 6. The heating element of claim 1 wherein 5 CTOSS'SCCUOH of said said ceramic core is comprised of several sections of 

1. A heating element suitable for horizontal suspension by support at its opposite ends within a furnace operative in a predetermined temperature range comprising: an elongated ceramic core having a plurality of longitudinal channels formed in the peripheral surface thereof and including: at least one elongated unitary core secTion having flat end surfaces oriented in a plane perpendicular to the longitudinal axis of said core; said at least one elongated core section when supported as aforesaid at temperatures within said predetermined temperature range being predisposed to sag and to place the lower portion thereof into tension; electrical heating coils disposed longitudinally within said channels; electrical connecting means for connection of said coils at said opposite ends to an external electrical power source; a metal rod having the characteristic of retaining its tensile properties throughout said temperature range, said rod being disposed on the longitudinal axis of said core and extending beyond the ends thereof; retaining means including perforated end pieces having flat faces confronting the respective flat end faces of said ceramic core and having an outer diameter at least equal to the cross sectional diameter of said core; at least one compression spring disposed at at least one end of said rod and beyond said perforated end pieces; spring retaining means including fastening means engaging said rod for maintaining said at least one spring in compression; said rod being maintained in tension by compression of said at least one spring to thereby maintain the ceramic core continuously in compression throughout the entire cross section thereof during operation of said heating element at temperatures within said range whereby tensioning and subsequent cracking of said at least one elongated section is prevented.
 2. The heating element of claim 1 wherein said at least one compression spring comprises first and second compression springs disposed beyond respective end pieces at said opposite ends for providing a uniform compressive force to said core from said opposite ends.
 3. The heating element of claim 1 wherein said ceramic core is formed with a central bore extending longitudinally therethrough and said rod is disposed within said center bore of said housing.
 4. The heating element of claim 1 wherein said elongated ceramic core is formed of a plurality of similar sections maintained in rigid axial alignment by said compression thereof.
 5. The heating element of claim 1 wherein said electrical heating coils are formed in independently energizable sections disposed longitudinally permitting control of temperature axially of said core.
 6. The heating element of claim 1 wherein said ceramic core is comprised of several sections of like cross-section cemented together at the abutting ends thereof to insure transmission of said compressive force uniformly across the entire cross-section of said core. 